cancel_message will probably lead to memory leaks

[tomaka]

Example situation:

• Process A send a TcpOpen request.
• Process B handles the message, allocates the TCP connection, returns the id.
• Process A in the meanwhile cancels the TcpOpen.
• Since process B isn't notified of the cancellation, we just leak the TCP connection.

[tomaka]

I wish I knew more about distributed systems or actor models here, but I don't think there's a generic solution to that problem. The TCP interface should be changed to allocate IDs on the sending side.